Duskey, I did have one party where two of the four guests stayed a bit after midnight (24:00).
Just a bit before midnight, I saw the message come up telling me the party was a "Blast," (so I guess the "party" was over, my Sim got the "Threw a Great Party" moodlet), but two of the guests stayed for roughly a half-hour before the "It's late, I need to leave"-type of message came up and they skedaddled.
I've never seen the party status message appear after midnight. But that's just my experience.
-- Happytime